The present invention relates to centralizers for centering pipe strings in wellbores, and, in particular, a casing centralizer.
In the drilling of wells, such as those for oil and gas, a string of tubulars is threaded together to form a drillstring having a drill bit mounted on the distal end. The drill bit is rotated either from the earth""s surface by rotating the drillstring of tubulars or by a downhole motor.
To enhance such rotary well drilling operations, numerous tool have been developed for mounting and use at sub-surface locations in the drillstring. One such tool is a centralizer. A centralizer contacts the borehole wall and effectively serves as a radial bearing or lateral support for the rotating drillstring in the borehole. By holding the drillstring against lateral forces or radial movement, the centralizer acts along the unsupported column length of the drillstring to prevent buckling as well as preventing excessive wear of the drillstring against the borehole wall. The centralizer also reduces the bending stresses induced by movement of the drillstring.
With the development of casing drilling, where the casing is used as the drill string and remains downhole as the wellborn liner, it is important that the integrity of the casing be maintained. To accomplish this, centralizers are used to prevent the drillpipe from contacting the borehole wall.
In conventional drilling, centralizers are usually formed by a tubular member with a plurality of outwardly extending fixed blades having wall contacting surfaces of hardened material that bear against or contact the sides of the borehole. The outwardly extending blades are usually mounted vertically or in a helical arrangement. The centralizers have threaded connections and are inserted into the drillstring at regular intervals by threading to the drillpipe threads in a conventional manner.
Casing drilling uses both special tubular sections and special threaded connections that ensure the integrity and gas tightness of the threaded connections. Centralizers that thread into the casing string are very expensive and are not convenient for use since they must be selected to fit exactly to the connection type being used.
Other prior art centralizers include locking collars to secure the centralizer to the drillstring. A locking collar uses set screws that engage into the material of the pipe. Through the locking collar, the centralizer is prevented from moving axially and from relative rotation on the pipe. However, a centralizer including a locking collar with set screws is relatively weak and sometimes cannot withstand the harsh drilling environment. In addition, the set screws damage the casing pipe, reducing its strength.
The present invention provides a centralizer and method for securing a centralizer to the drillstring. The centralizer is particularly useful where the drillstring is formed of casing. By utilizing the centralizer of the present invention, the casing string is centered in the borehole to reduce buckling forces and is protected from abrasion against the borehole walls without reducing the strength of the casing to undesirable levels. Centering the drillstring in the borehole is desirable to ensure that the borehole is drilled straight into the earth without unwanted deviation and to improve cementing. The centralizer also enhances hole cleaning by scraping against the borehole wall and by creating turbulence in the drilling mud passing thereby. A centralizer also acts to maintain the casing string spaced from the borehole wall to reduce differential sticking.
The centralizer according to the present invention, includes an outer sleeve and a retaining collar. The outer sleeve has a tapering, frustoconical inner surface and an outer surface including bearing surfaces such as, for example, blades or by application of weld beads. The retaining collar includes an outer frustoconical surface that substantially mates with the inner surface of the sleeve and a substantially cylindrical inner surface. The retaining collar is formed as a c-ring including a longitudinally extending open slit to allow for adjustment of its diameter and to permit mounting about casings of various diameter. Providing the retaining collar as a c-ring also provides that its inner diameter can be adjusted such that it will have good surface contact with the outer surface of the casing about which it is mounted.
A method is provided according to the present invention. In a broad aspect, a method for installing a centralizer on a joint of tubing comprises: providing a centralizer retaining collar having an outer conical surface, a substantially cylindrical inner surface and a longitudinally extending slit; providing an centralizer outer sleeve, having a tapering, generally conical inner surface substantially mateable with the outer surface of the retaining collar and an outer surface including bearing surfaces; mounting the retaining collar about the joint of tubing with the retaining collar inner surface adjacent the tubing outer surface; sliding the outer sleeve over the retaining collar such that their conical surfaces mate and the outer sleeve compresses the retaining collar into close engagement with the joint of tubing.
The method permits that the centralizer can be mounted over a joint of tubing, such as a casing joint, and can be held in place by frictional engagement between the outer sleeve, the retaining collar and the tubing outer surface. Forces are distributed evenly over the interface surface of the tubing and, as is particularly desirable for casing drilling, the casing integrity is not compromised by welding thereon or installation of set screws thereagainst.
In one embodiment, the retaining collar is selected such that its normal inner diameter is less than the outer diameter of the joint of tubing and the method further comprises expanding the retaining collar at the slit to fit over the joint of tubing. To enhance engagement between the parts, preferably, the contacting surfaces of the parts are cleaned prior to assembly to remove grease and/or oil therefrom.
The step of sliding the outer sleeve over the retaining collar such that their conical surfaces mate and the outer sleeve compresses the retaining collar into close engagement with the joint of tubing can be accomplished by heating the outer collar such that it expands prior to sliding the outer sleeve onto the retaining collar until it is wedged thereover. The outer collar, when permitted to cool, will shrink to further compress the retaining collar. In another embodiment, the outer sleeve is forced into wedging engagement over the retaining collar by application of force thereto as by hydraulics or hammering.